It is said that, if a pigment having higher durability in weather resistance, light resistance, water resistance, heat resistance, and so on as compared with a dye is made a nanoparticle, a color material having not only the same level of color characteristics as a dye in a tinting power, a coloring power, clearness, transparency, and gloss, but also excellent durability can be produced.
When a pigment nanoparticle is used in an ink jet ink, a color filter, a printing ink, a paint, or as a colorant for tonner and plastics, it is usually used by dispersing it in water or an organic solvent. To confer an intended color tone to a colored substance by using a pigment, a toning method in which pigments having different color tones are mixed is generally used. However, in many cases, a plurality of pigments which are used in this toning method have different chemical properties so that it is difficult to obtain a stable dispersion state; and thus, an advanced technology, high cost, and high energy are required to disperse a plurality of pigments stably.
To solve the problem as mentioned above, a toning method in which two or more types of pigment are made to a solid solution in a single particle has been known. As to the method to make a solid solution of pigments, there are several methods such as: a method in which a mixture of two or more types of pigment is subjected to a crushing treatment or a heat treatment as shown in Patent Document 1; a method in which a mixed pigment solution obtained by dissolving two or more types of pigment into a solvent is poured into an aqueous solvent as shown in Patent Document 2; and a method in which two or more pigment solutions having two or more types of pigment dissolved in each good solvent for respective pigments are simultaneously or sequentially mixed with one solvent capable of becoming a poor solvent to any of these solutions as shown in Patent Document 3.
However, in any of these methods, to produce a solid solution pigment nanoparticle having not only a uniform and fine particle diameter but also a uniform solid solution ratio in each particle has been very difficult. In the intended toning, a solid solution pigment nanoparticle having a uniform solid solution ratio in each nanoparticle has been wanted; and in addition, a method for producing a solid solution pigment nanoparticle having a controlled solid solution ratio has been eagerly wanted.
Applicant of the present invention provided, as shown in Patent Document 4, a method to produce a pigment nanoparticle wherein pigment particles are separated in a thin film fluid that flows between processing surfaces which are disposed in a position they are faced with each other; but a specific method for producing a solid solution pigment nanoparticle having a uniformly controlled solid solution ratio has not been disclosed.    Patent Document 1: Japanese Patent Application Publication No. 2004-528457    Patent Document 2: Japanese Patent Laid-Open Publication No. 2008-201914    Patent Document 3: Japanese Patent Laid-Open Publication No. 2009-74071    Patent Document 4: International Patent Laid-Open Publication No. 2009/008388